1. Field of the Invention
The present invention relates to a power train control apparatus having a torque distribution control function capable of varying the torque distribution to the front wheels and the rear wheels, or to the right wheels and the left wheels of a four-wheel drive vehicle or the torque distribution to the right wheel and the left wheel of a two-wheel drive vehicle and a slip control function capable of varying the braking force to be applied to each of the wheels in accordance with the state of slip of each of the wheels.
Hitherto, a torque distribution control (or a torque split control) and a slip control (or a traction control) have been conducted in order to improve the running stability of vehicles. The former control is a control to properly distribute the engine torque to each of the wheels in order to particularly improve the running performance on a curve or at the time of start of the vehicle. Namely, the torque distribution control performed as described above is conducted to correspond to the change in the load, to be applied to wheels, in accordance with the change in the attitude of the vehicle due to the change in the operation of the accelerator or the operation of the steering wheel. The slip control is conducted so as to control the slip ratio for the purpose of causing the state of the slip of the wheels to be taken place suitably. Namely, the slip control is conducted for the purpose of controlling the slip of the wheels caused from reasons (for example, running on a low-resistance road) which are individual from the operation performed by the driver.
As described above, the torque distribution control and the slip control have been respectively developed for the different purposes, and which are respectively conducted in different systems. However, since the above-described two control operations cause the torque for rotating the wheels to be changed, one of the control operation necessarily influences the other control operation. Therefore, an object of the control apparatus according to the present invention is to eliminate adverse effects upon the vehicle running caused from the interference between the two control operations.
Then, a variety of disclosed or suggested conventional technologies relating to the slip control and/or the torque distribution control will be described in terms of the structure, the operation and the problems involved therein prior to making the specific description about the thesis and the structure of the present invention.
Hitherto, a variety of technological ideas, for distributing different torque levels to the four wheels in, for example, a four-wheel drive vehicle (abbreviated to "a 4WD vehicle" hereinafter), has been suggested. For example, a technological means has been disclosed in Japanese Patent laid-Open No. 60-248440 in which the output from the engine is distributed to each of the wheels by a center differential gear and individual brake means is provided for each of the four wheels in order to independently control force for braking the wheels. As a result, when slip takes place in each of the wheels due to the state of the road, braking force is applied to the wheel which is being slipped. Thus, the force for rotating the wheel, which is being slipped, can be reduced. When the wheel, which is being slipped, is braked as described above, the difference in torque takes place in each of the other wheels. Therefore, a proper force for operating each of the four wheels can be obtained.
Since a vehicle is positioned in contact with the ground with the four wheels thereof, performance about the allowable slip is restricted by a frictional force of the tire defined by a product (.mu..multidot.W) of the friction coefficient .mu. of the road and a load W applied to the tire (the wheel). If the sum of the vectors of the components of the force applied to the tire, such as the rotating force, the braking force, centrifugal force and the like, is out of the above-described tire frictional force, a stable running performance of the vehicle cannot be obtained.
When the load W applied to the tire is shifted due to a longitudinal or lateral acceleration generated during the operation of the vehicle, the load applied to the four wheels becomes different from each other, causing the ability of the tires supporting the action of the vehicle to be also different from each other. In particularly, when the friction coefficient .mu. from the road is low, the influence of the load W upon the vehicle running becomes critical.
Therefore, the above-described 4WD vehicle achieves stable running performance even if the road has the low friction coefficient .mu. by properly distributing the wheel rotating torque to the four wheels and thereby reducing the load to be applied to each of the tires.
However, if the four wheels are coupled to one another by a center differential gear, the same wheel rotating torque is distributed to the front wheels and the rear wheels. As a result, the wheel to which a relatively light load is applied can be easily slipped. Therefore, performance of the vehicle is restricted by the tire whose ability has deteriorated, while the performance of the tire, whose load has been enlarged, cannot be exhibited.
However, when the 4WD vehicle of the type described above is arranged such that different torque is distributed to, for example, the front and rear wheels in order to overcome the above-described problem, each of the wheels can further effectively exhibit the road gripping performance. However, a structure, constituted such that the above-described torque distribution is changed by the adjustment of the strength of the connection (the frictional force) established in a clutch disposed in the above-described center differential gear, necessarily causes a great engine output to be directly distributed to the clutch. It leads to a problem in that the size of the clutch portion is enlarged and thereby the limitation occurs in mounting the clutch device. Therefore, the weight of the clutch device must be enlarged and the cost is thereby raised.
However, the above-described conventional technology, which is structured such that the brake device is provided for each of the front wheels and the rear wheels so as to be independently operated and the wheel which is being slipped is braked when the front wheel or the rear wheel is slipped, causes the vehicle speed to be reduced. Therefore, the running performance of the vehicle deteriorates, and particularly the accelerating performance of the same deteriorates.
Therefore, a torque split control system was suggested recently which is structured in accordance with the following knowledges: the state in which different torque is distributed to the front wheels and the rear wheels or to the right wheels and the left wheels is necessary takes place mainly in two cases, that is, a case in which the friction coefficient .mu. of the road is a small level and the running mode is the acceleration mode in which the wheel load is shifted due to the acceleration, and another case, that is, a turning mode in which centrifugal force is applied to the same wheel in addition to the wheel rotating force. Since there is, in usual, a great surplus torque in the engine output in the state described above, a certain engine output loss is permitted. Therefore, even if the torque split control is conducted, the vehicle does not encounter the deceleration in the vehicle speed. It leads to a fact that the necessity of providing a mechanism, which directly varies the amount of the torque to be transmitted, in the torque transmission passage can be eliminated.
The torque split control apparatus of this type, for use in a 4WD vehicle arranged such that the engine output is, as described above, transmitted to each of the four wheels, comprises: engine-output control means for controlling the change in the engine output; and braking force control means for controlling the independent change in the braking force to be applied to the front and rear wheels or the right and the left wheels. The operation of the engine-output control means and that of the braking force control means are controlled by torque distribution change means whereby the torque distribution to four wheels can be changed by the action of the braking force to be applied to any of four wheels by the braking force control means and by increase in the engine output, which corresponds to the above-described braking force, said increase being conducted by the engine output control means.
The above-described torque split control apparatus for use in the 4WD vehicle is arranged in such a manner that the front and rear wheels or the right and the left wheels whose torque is intended to be reduced is applied with a predetermined braking force by the braking force control means, while an engine output which corresponds to the torque reduced due to the braking force thus applied is distributed to the other wheels by the engine-output control means. As a result, the total engine output including increase in the engine output can be properly distributed to the front and the rear wheels. The torque, to be transmitted to the wheel on which the braking force acts, is reduced by the quantity which corresponds to the applied braking force. Therefore, torque which actually acts on the road from all of the wheels is the same torque before the above-described torque distribution is conducted, that is, before the braking operation. Namely, only torque to be transmitted to the wheel which is braked is reduced and torque to be transmitted to the wheel which is not braked is relatively enlarged.
As a result, the road-gripping force of the wheel which is being slipped is enlarged, causing the slip of this wheel to be prevented. Therefore, the running stability of the vehicle can be improved.
Furthermore, there is a so-called slip control (a traction control) system for controlling the driven wheels of a 4WD vehicle arranged similarly to the above-described torque split control system. If the wheel slips excessively on a road, the vehicle cannot usually run with the road gripped by its wheels. Therefore, proper running characteristics cannot be obtained due to a drift-out caused from wheel spins. In this case, a slip control apparatus for controlling the slip of the driven wheel with respect to the road is used, the slip control apparatus being arranged in such a manner that the brake device provided for the vehicle is operated similarly to the case where the above-described split control is conducted or the engine output is reduced for the purpose of preventing the occurrence of the slip of the driven wheels.
When the slip of the driven wheel on a road is prevented by the slip control apparatus of the type described above, it is necessary for the state in which the driven wheel slips on the road by a quantity exceeding a predetermined level to be detected. In addition, it is necessary for the vehicle speed to be first detected in order to determine a slip ratio or the like which serves as a reference of the driven wheel which is being slipped by a quantity exceeding a predetermined level. The vehicle speed can be relatively easily detected in accordance with the circumferential speed of the follower wheel which does not frequently slip in the case of a 2WD vehicle arranged such that only the front or the rear wheels are driven. However, it is difficult to detect it in the case of the 4WD mode of the 4WD vehicle where both the front wheels and the rear wheels can be driven as described above since there are no follower wheels.
A slip control apparatus for use in a 4WD vehicle has been suggested as disclosed in, for example, Japanese Patent Laid-Open No. 62-289429 in order to overcome the problems taken place when the slip ratio in the 4WD vehicle is detected. The slip control apparatus is arranged in such a manner that: the circumferential speed of each of the four wheels is obtained; the state in which each of the wheels slips by a quantity exceeding a predetermined level with respect to a road is detected on the basis of the thus-obtained circumferential speed; and the vehicle speed is specifically estimated on the basis of the minimal circumferential speed of the wheels.
The slip control apparatus for a 4WD vehicle, arranged such that the state in which each of the wheels slips on a road can be detected and the vehicle speed can be estimated, performs a control when a wheel slip with respect to a road by a quantity exceeding a predetermined level is detected, the control being arranged to reduce the torque for rotating the wheel from which the slip state exceeding a predetermined level has been detected in order to cause the slip ratio or the quantity of the slip of the wheel from which the slip state exceeding a predetermined level has been detected. The target value in the control described above is determined regardless of the number of the wheels from which the slip state exceeding a predetermined level has been detected.
In general, the number of the wheels of a 4WD vehicle from which the slip on a road has been detected becomes the number which corresponds to the state of the vehicle running or the road condition (the friction coefficient of the road or the like). The greater the number of the wheels which slips by a quantity exceeding a predetermined level, the larger the influence upon the running stability of the vehicle is. Therefore, it is preferable that the target slip ratio or the quantity of the slip be determined to a low level and thereby the running stability of the vehicle be first secured at the time of the slip control in inverse proportion to the number of the wheels from which the slip state exceeding a predetermined level has been detected. On the contrary, the target slip ratio or the quantity of the slip be determined to a high level and thereby the running characteristics of the vehicle such as the accelerating performance or the cruising performance be first secured at the time of the slip control in inverse proportion to the number of the wheels from which the slip state exceeding a predetermined level has been detected.
Recently, a precise slip control apparatus for a 4WD vehicle has been disclosed which is arranged to act when the slip state exceeding a predetermined level has been detected from any of the wheels of the 4WD vehicle, the slip control apparatus being provided with torque control means for changing the torque acting on the wheel from which the slip state exceeding a predetermined level has been detected. Thus, the slip ratio or the quantity of slip of the wheel is caused to agree with a predetermined target value. Furthermore, a target slip ratio or the same of the quantity of slip are determined in accordance with the vehicle running state or the road condition so that desired running characteristics can be obtained with the running stability of the vehicle secured substantially.
As described above, the slip control (the traction control) of the driven wheel and the torque distribution control (the torque split control) of the same contribute to achieve the stable running of the vehicle and thereby improve the driving stability and the accelerating performance. However, the inventors of the present invention found a fact that a vehicle provided with the above-described two systems can arise the following problem when the two systems are simultaneously operated:
For example, there are two case in which the above-described slip control operation is obstructed by the torque distribution (the torque split) control conducted in accordance with the quantity of movement of the vehicle body or the quantity of the operation of the driver and another case in which the same is not obstructed. Therefore, the torque distribution control must be conducted in accordance with the state of the vehicle. That is, when there is a fear that the slip control is obstructed and thereby the vehicle running stability deteriorates, the torque distribution control must be cancelled. However, if the torque distribution control is conducted and only the slip control is conducted in the other case, the accelerating performance of the vehicle deteriorates inevitably.
On the other hand, when the torque distribution control is conducted as described above in accordance with the quantity of movement of the vehicle body or the quantity of the operation by the driver, the performance of each of the tires for each of the front and rear wheels can be caused to exhibit maximally as described above. However, if the force to rotate the wheel is enlarged by a level exceeding the limit of the performance of the tires which becomes different depending upon the road condition (in particular, the friction coefficient .mu. of a road), the wheel spin cannot be prevented. For example, the limit difference in the performance of tires due to the state of the action of the vehicle is reduced on a road whose friction coefficient .mu. is a low level such a snowy road. In this case, the increase in the distribution of the wheel rotating force can cause the spins of the wheels depending upon the positions of the wheels or the number of the wheels.
In order to prevent the spin taken place in the above-described case, it is effective to conduct the above-described slip control so as to reduce the torque acting on the wheel. However, if the above-described torque distribution control is rapidly shifted to the slip control, the wheels from which the rotating force must be reduced lose their controlling ability with respect to the road. Furthermore, the rapid change in the distributed quantity of the wheel rotating force causes the vehicle to move instably.